Rinse the brown lentils under cold water, then set aside. In a large pot, bring the vegetable broth to a boil. Add the lentils, reduce heat to low, and simmer uncovered for 20-25 minutes until tender but still holding their shape. Drain any excess liquid and set aside.
While the lentils cook, dice the carrots and onion. Heat 2 tablespoons of olive oil in your ovenproof skillet over medium heat. Add the diced vegetables and sauté for about 8-10 minutes, stirring occasionally, until they are fragrant and lightly caramelized, with a sweet aroma filling the kitchen.
Stir the cooked lentils into the sautéed vegetables along with thyme, salt, pepper, and a splash of vegan Worcestershire sauce if using. Cook everything together for another 5 minutes, allowing the flavors to meld and the mixture to become fragrant and hearty.
Meanwhile, peel and chop the potatoes. Place them in a large pot and cover with salted water. Bring to a boil over high heat, then reduce to a simmer for 15-20 minutes until the potatoes are fork-tender and fluffy.
Drain the potatoes thoroughly and return them to the hot pot. Add vegan butter or olive oil and mash with a fork or potato masher until smooth and creamy. Stir in plant-based milk gradually until reaching your desired consistency. Adjust seasoning with salt and pepper.
Preheat your oven to 200°C (400°F). Spread the lentil and vegetable mixture evenly in the skillet or casserole dish. Then, dollop and spread the mashed potatoes on top, smoothing with a spatula or fork to cover the filling completely. For a rustic look, create some ridges with the back of a fork.
Bake in the preheated oven for 20-25 minutes until the top is golden brown and slightly crispy around the edges, with bubbling filling around the sides. The aroma of caramelized vegetables and baked potatoes will fill your kitchen.
Remove from the oven and let sit for 5-10 minutes to allow the filling to set. Serve slices directly from the skillet, garnished with fresh herbs if desired, and enjoy the comforting, hearty flavors of this plant-based classic.